The Universal Mobile Telecommunication System (UMTS) is one of the third generation mobile communication technologies designed to succeed GSM. 3GPP Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to improve the UMTS standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, lowered costs etc. The UMTS Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS system and evolved UTRAN (E-UTRAN) is the radio access network of an LTE system.
As illustrated in FIG. 1a, an E-UTRAN typically comprises user equipments (UE) 150 wirelessly connected to radio base stations (RBS) 100, commonly referred to as eNodeB. In such a wireless communication system, it is desirable to reuse as much of the radio resources in each cell 110 as possible to achieve good spectral efficiency. The radio resources may be e.g. the time slots, the frequencies, the code resources and the transmission power of the radio base stations and the user equipment. In an E-UTRAN, downlink multiplexing is a combination of Time Division Multiplexing (TDM) and Frequency Division Multiplexing (FDM) and resources are thus shared both in the time domain and the frequency domain. The scheduling unit 120, commonly co-located with the eNodeB, controls the allocation of these shared radio resources among the UEs at each time instant, both in uplink and in downlink.
However, whenever a resource is reused this may lead to interference (intra-cell or inter-cell interference) between the links utilizing this particular resource. The scheduling principles thus need to take the interference into account. The scheduling principles, as well as which resources that are shared between UEs, differ depending on the radio interface characteristics, e.g. whether uplink or downlink is considered and whether different users transmissions are mutually orthogonal or not.
Various approaches to manage the interference are known. One possibility is to refrain from using some fraction of the available resources in each cell. By coordination of the resource usage in multiple cells an acceptable interference level can be achieved. Some examples of this approach are frequency reuse, static inter-cell interference coordination (ICIC), and fractional load. A problem with these approaches for managing interference is that they restrict the resources available for scheduling, thus reducing the spectral efficiency as resources are not used optimally in each cell.
Another approach relies on more actively selecting which UEs that can access a particular resource based on channel state information for these UEs. Of all possible UEs, those who interfere the least with each other may be scheduled jointly. Such a scheme can be made possible by having knowledge of the average channel gain on all desired and some interfering links. Some examples are coordinated scheduled beamforming, multipoint/multicell coordinated scheduling and advanced dynamic ICIC. The problem of such a scheme is that it requires accurate measurements of the channel gain to both desired and interfering UEs, which also reduces the spectral efficiency as the measurements use resources that could have been used for payload. Improving the accuracy of the measurements by assigning orthogonal sounding pilots to the different links is also done at the cost of poorer spectral efficiency.
An even more advanced approach, which is studied for advanced E-UTRAN in 3GPP, utilizes coordinated transmission (or reception) from multiple RBSs to effectively “null” the interference between UEs served by these RBSs, and may also simultaneously to (from) multipoints coherently transmit (or receive) signals to improve performance. One such arrangement is called a coherent CoMP system, which is a network of spatially distributed antenna nodes connected to a common CoMP RBS, that provides wireless service within a geographic area. FIG. 1b illustrates the basic concepts of a CoMP system. The CoMP RBS 130, coordinates the function of all the antenna nodes 160, and the scheduling unit 120 typically co-located with the CoMP RBS, applies different weights to the transmission antennas of the distributed antenna nodes 160 in order to serve the UEs 150. Applying different weights to the transmission antennas implies that the signals to be transmitted on the different antennas are multiplied by different weights, thereby adjusting the phase and/or the amplitude of the signals, in order to shape the overall antenna beam in the wanted direction. This may also be expressed as applying a pre-coding vector to the signal to be transmitted or to the transmitting antenna. The area covered by each antenna node 160 is referred to as a sub cell 170, and the area covered by a CoMP system is referred to as a CoMP cell 140. Typically the UE 150 receives signals from more than one antenna node 160 in a CoMP cell 140.
The problem with such a system is that instantaneous channel knowledge is required to achieve useful gains. This makes this approach very costly in terms of the necessary amount of feed-back to maintain enough channel knowledge, especially in high mobility scenarios.